Impedance matching is widely used in the transmission of signals in applications such as industrial, communication, video, medical and test and measurement markets. For example, impedance matching is used in the transmission of a signal through a co-axial cable for delivery to an ultrasound transducer for use in ultrasound imaging medical devices. Referring to FIG. 1 there is shown a transmission circuit 10 of the prior art for use in ultrasound transmission application. The circuit 10 comprises an ultrasound transmitter 12, which is typically an integrated circuit chip generating a driving signal and supplying it to a back termination resistor 14, which is connected to a co-axial transmission cable 16, which is connected to an ultrasound transducer 20. The transmission cable 16 is typically long (on the order of 2 meters) and is usually a 75 ohm cable. The impedance of the back terminating resistor 14 is matched to that of the cable 16. Thus, the resistor 14 is also on the order of 75 ohms. The use of a resistor 14 having substantially the same impedance as the cable 16 results in maximum signal transfer, and eliminate or minimizes signals reflected from the transducer 20 to reduce or eliminate ringing.
The advantage of using only a back terminated resistor 14 is that it adds only one resistor per driver and the terminating resistor 14 consumes little power. In addition, the series termination adds no dc load to the driver circuit 12 and offers no extra impedance from the signal line to ground. The disadvantage of the use of a resistor 14 connected in series termination fashion is that it is difficult to tune the resistance of the resistor 14 so that the received signal amplitude (after the first reflection) falls within the noise level. In addition, most ultrasound driver circuits 12 are non-linear. Thus, the output impedance would vary with the logic state of the device 12. Furthermore, there can be wide variation in the transmitter chip 12 from one driver circuit 12 to another driver circuit, depending upon the operating temperature range, power supply voltage range and other operating conditions. Thus, it is difficult to select a single value for the resistance of the resistor 14 for all driver circuits 12.
To overcome the foregoing disadvantages, the resistor 14 can be placed in the transmission driver circuit 12, and integrated with the integrated circuit device. Thus, as shown in FIG. 2, there is disclosed another transmission circuit 30 of the prior art in which the matching resistor 14 is added to the driving circuit 12. As a result, the output impedance of the driver circuit 12 can be matched to the transmission media, or the cable 16. Furthermore, the output impedance can be matched for the case where the signal in the driver circuit 12 goes low as well as goes high. However, if the resistor 14 is integrated with the driver circuit 12, the resistor 14 is subject to process variations in the fabrication of the driver circuit 12. For example, current semiconductor processing technology results in process variation of as much as ±30% in variation, resulting in a spread of ±30% in the output impedance of the driver circuit 12 and ±15% in the output voltage.
In another prior art circuit 50 shown in FIG. 3, the circuit 50 uses pre-driver inverter power supplies to switch the output driver circuit 12. The circuit 50 controls the gate-to-source voltage resulting in the linear resistance forced to match the resistance of the external line. The NMOS transistor M2 of the driver circuit 12 is driven from Vlow to Vrn, by the first pre-inverter driver circuit, and the PMOS transistor M1 of the driver circuit 12 is driven from Vrp to Vhi, by the second pre-inverter driver circuit. However, such circuit 50 suffers from difficulty in creating and maintaining the precise voltages required.